The domains with large numbers of TXT records are also used in DNS DDoS amplification attacks. Spoofed UDP requests to domains that have a large number of TXT records are used to slam other sites. In the past I would transparently strip the TXT records when I ran public DNS recursive resolvers nobody noticed except the botters but some here may be activated. Some domains with a lot of dangling records:

    for i in $(echo "ycombinator.com 500px.com box.com ebay.com google.com hm.com lenovo.com nordstrom.com realtor.com tmz.com wired.com");do echo -en "${i}:  ";dig +short +nocookie -t TXT "${i}"|wc -l;done|sort -rn -k2
    nordstrom.com:  39
    lenovo.com:  38
    realtor.com:  36
    ebay.com:  36
    hm.com:  34
    box.com:  28
    wired.com:  27
    tmz.com:  22
    500px.com:  17
    ycombinator.com:  13
    google.com:  13

Ebay used to be in first place, not sure what changed.

In unbound.conf:

    local-zone: ycombinator.com typetransparent
    local-data: 'ycombinator.com. TXT "[ddos redacted]"'

after the changes:

    dig +short +nocookie -t txt ycombinator.com
    "[ddos redacted]"